The Invention relates to a laser light source, having a wide-band amplifying, narrow-band tunable medium, especially a semiconductor laser light source, which is characterized by the possibility of tuning the Laser wavelength, at least in broad ranges of the laser gain curve, without mode jumps, and at the same time by high optical stability. Possible areas of application for a light source of this type include optical spectroscopy.
Tunable light sources having a semiconductor laser as the active element are known in the art in many variations. Included among these are those light sources that enable continuous tuning in a more or less broad spectral range, or that permit, with the acceptance of wavelength jumps, the adjustment of any wavelength within a given interval.
To enable continuous tunability over a given wavelength range it is necessary, during the tuning of the emission wavelength of the laser, for the optical path length in the resonator cavity to change correspondingly and simultaneously, such that in single-mode operation the number of the oscillating longitudinal mode in the given range is maintained and mode jumps are prevented. Light sources of this type are known as monolithic components and as hybrid arrangements.
Monolithic components are characterized by their compactness and by the fact that the tuning of by the emission wavelength and the corresponding change of the optical path length within the resonator cavity can be synchronized using electronic means alone. The range of continuous tunability, however, is at least 2 to 5 times narrower than the laser gain curve. Up to now, a usable, continuous tuning range of 9 nm with a central wavelength of 1555 nm (corresponding to approx. 0.6%) has been achieved/T. Wolf, S. Illek, J. Rieger, B. Borchert, M. C. Axnann: Extended Continuous Tuning Range (over 10 nm) of Tunable Twin-Guide Lasers, Technical Digest Conference on Lasers and Electro-Optics (CLEO""94), Anaheim, May 8-13 1994. CWB1/.
Hybrid arrangements are comprised principally of a laser diode that is preferably highly antireflection-coated on one side, so that it will no longer act as a resonator, to the greatest extent possible, and an external resonator section that permits a wavelength-selective, tunable back coupling of the emitted light onto the antireflection-coated laser facet. This enables tuning over the entire gain curve of the laser. In principle, the adjustment of the resonator cavity length can occur separately from the selection of the back-coupled spectral range, without taking additional parameters into consideration. Up to now, however, only relatively complicated devices designed for use in implementing this adjustment more or less automatically have been known in the art. The following three solutions represent the state of the art.
The first solution was described by Favre, et al./ F. Favre, D. Le Guen, J. C. Simon, B. Landousies: External-Cavity Semiconductor Laser With 15 nm Continuous Tuning Range, Electronic Letters, Vol. 22. No. 15, Jul. 17, 1986, 795-796/. The light emanating from the antireflection-coated laser facet is collimated through an optic and travels, as a parallel bundle, at a specific angle, to a flat diffraction grating. The light which has been diffracted back into the direction of incidence returns, after traveling through the optic, to the laser. The special characteristic of this solution consists in that the rotation of the grating (tuning of the selected range) via two translational elements and one coupling rod is mechanically linked to the adjustment of the resonator cavity length, so that a continuous tuning of the laser wavelength is possible over 1.2% of the center wavelength. The positioning of the grating on a piezo translator permits small deviations to be corrected.
A significant disadvantage of this solution, as with many other arrangements, consists in that the laser light is naturally coupled back as a collimated bundle of rays. The result of this is that, in addition to the two degrees of freedom that are necessary for jump-free wavelength tuning (displacement and rotation of the grating), two additional degrees of freedom, which are not necessary for the adjustment of any base parameters, must be maintained in a highly sensitive state, in the optimum position. One of these is the tilting of the beam path perpendicular to the direction of dispersion for the grating, and the other is the displacement of the laser chip along the optical axis in relation to the collimator, in order to achieve the necessary precise imaging of the laser facet. Since the optically effective laser chip facet is very small, precise requirements are placed upon the precision and stability of the mechanics. This is complicated by the fact that within this two-dimensional possibility for adjustment there is only one optimum position and only one base parameter, namely the radiant power produced, that can be used as a scale. This disadvantage also applies correspondingly to the two following examples.
In/ W. Fuhrmann, W. Demtrbder: A Continuously Tunable GaAs Diode Laser With an External Resonator. Appl. Phys. B 49, 29-32 (1989), another hybrid arrangement is described. The connection between the selected spectral range and the optical path length in the resonator cavity is produced here via an electronic control device; the controlling element used to adjust the effective resonator cavity length is a Brewster plate that can be rotated via a galvanometric device. A total tuning range of 1.8% of the average emission wavelength, but only 0.014% of this continuously, is achieved.
EP 0335691.A1, H 01 S 3/08 contains a solution in which the change in resonator cavity length is achieved using a piezo translator alone. If tuning is to take place continuously over more than a fraction of a mode interval, an electronic control device is also required. Due to the limited adjustment path of the piezo translator, the continuously tunable range is also small in this arrangement.
The mechanical construction of the first solution alone permits continuous adjustment of the wavelength over a wide range, due to the long adjustment path for the resonator cavity length. The mechanics of this solution, however, naturally permit only slow tuning. The two other solutions permit continuous tuning over only narrow ranges.
For wavelength selection in lasers having a broad-band medium that can be stimulated, especially dye lasers, another arrangement is known in the art, which is treated in different variations in DE-AS 2051328, H 01 S 3/08 and the associated supplementary patent DE-OS 2236505, H 01 S 3/08. In these, the selection of wavelength is achieved primarily in that within the resonator cavity, the light is focused in a pinhole diaphragm, and behind this pinhole diaphragm an optic having a high degree of longitudinal chromatic aberration and a low degree of aperture aberration is positioned, such that in combination with one of the resonator cavity mirrors an imaging back into the pinhole diaphragm occurs without substantial loss, for only a narrow wavelength range. The tuning is effected by shifting the optic along its optical axis. In order to increase the selectivity, the optical axis of the selection arrangement is either shifted in relation to the geometric axis of the medium that can be stimulated or shifted to form an angle with it. The optic having the high degree of longitudinal chromatic aberration can be combined with the associated resonator cavity end mirror to form a single component, a Fresnel zone plate.
The contents of this patent relate exclusively to the selection of the wavelength. No reference is made to the behavior of the modes in the resonator cavity or to the stability of the arrangement.
The state of the art in the technology of increasing adjustment tolerances for lasers having external resonator cavities is determined principally by two solutions: The first solution is described in / P. Zorabedian and W. R. Trutna, Jr.: Interference-Filter-Tuned, Alignment-Stabilized, Semiconductor External-Cavity Laser, OPTICS LETTERS / Vol. 13, No. 10 (1988), pp 826 . . . 828/. To enable adjustment-tolerant back coupling of the laser beam, a cat""s eye reflex reflector (collective lens with mirror in its focal plane) is used. As the selective element, an interference filter is found in the parallel ray path within the resonator cavity. To enable tuning of the laser wavelength, this filter is mounted such that it can be tilted. The coupling out of the usable beam follows from the facet of the laser chip that faces away from the external resonator cavity.
A further possibility for constructing an adjustment-stable laser having an external resonator cavity is contained in EP-O 525 752 A1 H 01 S 3/1055. In this case, as before, a cat""s eye reflex reflector is used; in principle, however, its effect is limited to a single coordinate. With a specially designed combination of prisms an d cylindrical optics for formation of the beam, and using a diffraction grating as the reflector, the result is that an imaging of the laser facet onto the grating occurs only perpendicular to the direction of dispersion. In the direction of dispersion, however, the bundle of rays that reaches the grating is extensively parallel and relatively broad. The result is thus that the grating can be used without limits to tune-the laser wavelength, on the other hand, the positioning is largely tolerant to a tilting of the grating, perpendicular to the direction of dispersion.
The latter two solutions increase considerably the tolerance to tilting of the back-coupling beam path. There still remains, however, one degree of freedom that is not stabilized, namely the shifting of the laser chip in relation to the collimator along the optical axis, for the purpose of focusing the image of the laser facet onto the same. In addition, these solutions contain no wavelength tuning that is free from mode jumps.
Laser light sources that are continuously tunable over a broad range are necessary in the field of optical spectroscopy, among other areas. Their importance results, for example, from the fact that a mode-interval for traditional resonator cavity lengths amounts to approximately the width of one spectral line, thus, in the case of discontinuous tuning, correspondingly broad wavelength ranges will be jumped over.
Devices that are already available, all of which operate on the basic principle described by Favre, et al. (see above), contain, due to the high degree of adjustment sensitivity of the arrangement, means, such as active controls and/or a massive construction, that make them quite costly. It would be desirable to have a light source of this kind which would require considerably less expense for stabilization of the system, and would enable a more rapid tuning of the wavelength or a wavelength modulation.
The object of the invention consists in the creation of a semiconductor laser light source in which a high degree of stability is achieved as a result of the optical concept, and which, in addition, offers the possibility of jump-free tuning of the laser wavelength at least over broad partial ranges of the gain curve of the semiconductor material, due to the fact that the necessary connection between the selection of the back-coupled spectral range and the optical path length in the resonator cavity is provided in a simple and largely non-mechanical manner.
This object is attained in accordance with the invention with the characteristics of claims 1 and 11. The tunable semiconductor laser light source is formed from a hybrid arrangement, comprised principally of the following:
a laser chip,
an optical system that is positioned such that it faces the emission facet of the laser chip, and possesses a high degree of longitudinal chromatic aberration,
a first resonator cavity end mirror, which preferably is formed from the facet of the laser chip that is positioned such that it faces away from the optical system,
and a second resonator cavity end mirror, designed as a reflector.
The high degree of longitudinal chromatic aberration is achieved via an appropriate configuration and via the selection of imaging scale for the optical system.
The term xe2x80x9cemitting channelxe2x80x9d is used to refer to the active zone of the preferably highly antireflection-coated laser chip, in which the stimulated emission appears. The ends of the emitting channel, at which the channel reaches the surface areas of the laser chip, are generally referred to as (naturally not clearly defined) xe2x80x9claser diode facetsxe2x80x9d or xe2x80x9cfacets of the laser chip,xe2x80x9d or specifically as xe2x80x9claser diode emission facetsxe2x80x9d or xe2x80x9cemitting facets of the laser chip.xe2x80x9d In the corresponding connection, the laser diode emission facet is the laser facet that is advantageously highly antireflection coated and positioned such that it faces the optical system. The laser diode emission facet, to a certain extent, represents a window to the emitting channel. In addition, the radiation that proceeds from the laser chip is more or less astigmatic, depending upon the type of semiconductor laser used. The laser diode emission facet is thus not simply a luminescent surface. The radiation is coupled out of the chip from the emitting channel via the laser diode emission facet, is directed to the external portion of the resonator cavity arrangement, and finally, with a high degree of longitudinal chromatic aberration, is back coupled into the emitting channel via the laser diode emission facet. The high degree of longitudinal chromatic aberration of the imaging, in connection with the laser diode emission facet, which acts as a space filter, produces the selectivity of the arrangement.